Project 1 aims to characterize the accumulation of fibrillar cerebral amyloid-beta (AP) over the age range from the time of onset of Ap deposits to the time when Ap levels approximate those of symptomatic Alzheimer's disease (AD). Project 1 proposes a longitudinal [^^C] PIB imaging study in the cohort of the Adult Children Study (ACS). PET PIB scans will be done on ail ACS participants every two years. The data to be obtained will greatly expand knowledge about preclinical AD and the clinical correlates of Ap accumulation. The long term objective of this effort is to obtain data that will facilitate and foster the design and conduct of clinical trials of anti-Ap therapies for the primary prevention of symptomatic AD. In particular, it aims to help determine the optimal time for intervention in such a trial. Specific Aims are: 1. Obtain longitudinal [C] PIB PET scans every 2 years in cognitively normal ACS participants to identify the conversion rate from no or minimal fibrillar Ajj deposition to appreciable AP accumulation, defined by mean cortical binding potential (MCBP) for PIB >0.18, and determine whether this rate is affected by age, APOE genotype, and parental history of AD. 2. Determine the rate and trajectory of change in Ap accumulation in ACS participants, both those with appreciable Ap deposits at their baseline [^^C] PIB PET scan and those lacking Ap deposits at baseline. 3. Examine associations and interactions of cognitive and brain reserve variables (e.g., normalized whole brain volume; regional atrophy; occupation; educational attainment) with longitudinal cognitive performance, obtained in the Clinical Core, in participants with and without Ap accumulation. 4. Correlate findings from Project 1 with appropriate data from Projects 2, 3, and 4 and utilize the Administration (e.g., budgetary oversight), Clinical (e.g., source of participants; longitudinal clinical and cognitive data), Biomarker (e.g., cerebrospinal fluid levels of AP42 in Project 1 participants imaged for Ap deposits), and Data Management and Biostatistics (e.g., data analyses) Cores of this program project grant.